The present disclosure relates generally to electron spectroscopy and more particularly to an electron spectroscopy system including a radio frequency cavity and multiple spectrometers.
The ability to control and engineer complex materials and nanostructures is essential for enabling an array of technologies including: solar-energy harvesting, solar to fuel conversion, heat recovery, and the development of novel photoactive nanostructured materials and nanoelectronics devices used in computing and internet infrastructure. It is widely recognized that a major bottleneck to progress is the scarcity of experimental data on the local properties, which has prevented the successful development and validation of advanced predictive models. Information about collective excitations, such as plasmons, and low-energy excitations, which involve holes and electrons in the valence and conduction bands, can be inspected in the low-energy loss region. However, such information is usually very difficult to obtain as it often involves multiple steps and collective modes that are frequently not accessible via optical devices. These different transient responses highlight the complexity of charge transfer dynamics at heterogeneous interfaces consisting of active plasmonic nanostructures, surface and dielectric medium over different frequencies and timescales. The traditional shortcomings in resolving these processes are exacerbated by the limitations in terms of temporal resolution and sensitivity using only a single or very few electrons in each electron gun pulse or shot with conventional electron microscopes.
Strongly correlated electronic materials are promising candidates to provide new functionality that cannot be found in conventional semiconductors and metals, in part because of their complex types of ordering. The tendency for their complex ordering, which leads to highly remarkable and important properties such as high-Tc superconductivity and giant magnetoresistance, arises because correlated matter very often exists close to several different types of ordered phases due to complex interactions between the orbital states of localized electrons and the distortion of ionic lattices. Understanding the fundamental excitations near the phase-transition threshold between different states is central to not only strongly correlated materials, but also the science to synthesize and process them. The collective excitations that emerge near these thresholds often exhibit complex spatiotemporal patterns that are different from conventional semiconductors or metals, such as density waves and macroscopic charge orders. While advances in ultrafast imaging technology are now at the level where these spatiotemproal pattern may be imaged using ultrashort coherent X-ray beams or with coherent electron beams on the ultrafast timescale, the still existing challenge is to correlate these spatiotemporal patterns with their underlying electronic structure evolution that is the primary source of their functionalities. However, the most advanced laser-based femtosecond (“fs”) angle-resolved spectroscopy for imaging band structures, namely ultrafast angle-resolved electron spectroscopy, is limited by its use of low energy electrons to two dimensional (“2D”) Fermi surfaces and Brillouin zones. Meanwhile, a major challenge in this field is now reconciling the emerging properties due to macroscopic entanglement between different electronic orderings since such transformations not only depend on the properties of 2D electronic distributions, but also the interaction of the density waves through structures across the layers.
Furthermore, electron energy-loss spectroscopy (“EELS”) is employed in high time-resolution transmission electron microscopy (“TEM”) as a new ultrafast probe for studying element-specific and local electronic processes, whereby it is desirable to reach a high overall signal strength as reproducible events in this regime typically correspond to less than a few percentage changes in the electron density and chemical bonding. The most challenging issue thus far preventing wide-ranging applications of ultrafast EELS is the presence of space-charge effect resulting in poor time and energy resolutions in TEM. The alternative solution to this is to use only one or few electrons in each electron packet complemented by a high-repetition rate (typically ˜MHz) to preserve the inherent resolutions (typically ˜0.6 eV-500 fs, limited mainly by initial stochastic velocity spread at photoemission) in ultrafast electron microscopes (“UEM”). Whereas this approach is ideally suited for imaging highly reproducible electronic processes, as in many applications in photon-induced near-field electron microscopy mode and has been applied to study band-edge renormalization at low-loss regions, a higher instantaneous dose is desired for imaging more chemically sensitive core-level spectroscopy. Not only is the cross-section in core losses much lower, many applications involving chemical transformations and bonding changes are also typically less reproducible, thereby demanding orders of magnitude improvements in the instantaneous dose.
In accordance with the present invention, an electron spectroscopy system and method are disclosed. In another aspect, an ultrabright and ultrafast angle-resolved electron spectroscopy system is provided. A further aspect of the present system employs an electron gun, a radio frequency cavity and multiple spectrometers. Yet another aspect uses spectrometers in an aligned manner to deflect and focus electrons emitted by the electron gun. Moreover, an ultrafast laser is coupled to an electron spectroscopy system. A bunch of monochromatic electrons have their energy compressed and reoriented in an additional aspect of the present system.
The present electron spectroscopy system and method advantageously enhance temporal and momentum resolution, and throughput to ultimately allow studies of individual nanostructures with full three-dimensional (“3D”) momentum resolution over an entire Brillouin zone. Such capabilities are currently not available in conventional electron-based spectroscopy systems. An advantageous foundation of the present ultrabright monochromatic electron beams is through active energy compression to preserve the throughput of the beam for materials research. Furthermore, the innovative present tandem spectrometer system achieves high momentum and energy resolution from 0.1 eV to 10 eV for low-loss and core-loss electron spectroscopy with fs-ps time resolution, which are not obtainable with conventional electron microscopes or spectrometers. The present system will extend ultrafast material spectroscopy studies to the realm of characterization of transient three-dimensional electronic structures with unprecedented sensitivity.
The high-throughput of the present system is the ideal probe for unveiling transient photochemical processes due to its high sensitivity to charge states and its more direct accesses to local electronic structures and hot electron dynamics for pinpointing the origins of these local, transient electronic processes. In addition, the local structure can be extracted by analyzing the extended energy loss fine structure, which is an analogue of extended X-ray absorption fine structure.
The present system advantageously provides full-scale 3D momentum spectroscopy by high-energy beams that penetrate the bulk of samples and the ability to sample large energy dispersion and momentum distribution. Beyond correlated electron physics, the high momentum and high time-resolution spectroscopy established through the present system provides new capabilities relevant to understanding an array of complex materials issues of broad interest, including studies of high-temperature superconductors, topological insulators, materials exhibiting photo-induced phase transitions, and novel correlated-electron-based switching devices. Furthermore, the present system has capabilities for in-situ, ultrafast characterization of transient processes and phases. This ability for precisely tracking material modifications of carbon-based materials into a variety of novel phases induced by irradiation is demonstrated in the distinctive core-level spectra of these different phases when the energy window is tuned to the carbon K-edge. An additional use of the present system is for analysis of emergent behavior of matters under extreme environments. Moreover, the development of the present RF-enabled system is a significant step towards reaching effective brilliance and time-resolution as free-electron laser X-ray beamlines for spectroscopy.
Conventional momentum-resolved spectroscopy typically requires X-ray or electron probes due to the low momentum transfer at optical frequencies. In comparison to some traditional laser-based electron spectroscopy systems, namely angle-resolved photoelectron spectroscopy and photoelectron electron microscopy, the present approach is highly complementary and excels in providing higher throughput enabling studies of materials processes where high-repetition-rate experiments are not feasible.
A further aspect of the present electron spectroscopy system employs adaptive and/or adjustable optics to optimize both time and energy compression. Another aspect provides at least two RF lenses or cavities, one before a specimen and one after the specimen. Still another aspect includes a programmable controller and associated software instructions which automatically optimize and/or adjust multiple lens in a laser based electron microscope while images are being produced for a specimen. These aspects of the present system improve both the time and energy resolution of the microscope and the detail of the image created therewith. Additional advantages and features can be found in the following description and appended drawings.